High electron mobility transistors (HEMTs) are excellent candidates for high power and high frequency applications. However, some undesired issues, such as electron trapping at surface states of HEMTs, affect the breakdown voltage, gate leakage current and current collapse, and could lead to degraded direct current (DC) and radio frequency (RF) performance. These issues hinder the advancement of GaN-based power and high frequency device technology. Degraded HEMTs show a significantly higher trap density, which can be confirmed by temperature-dependent drain-current sub-threshold swing measurements. Step-stress induced gate metal diffusions and notches formed along the gate edges on drain side or on both source and drain sides have been reported to be the causes of permanent device degradation in HEMTs.